Hydrocyclone having a high efficiency area to volume ratio

ABSTRACT

A high efficiency hydrocyclone for separating a dispersed liquid phase from a continuous liquid phase in a mixture includes a separation chamber having an effective separation portion L e  that begins where the slope or curve of the portion is less than 2°. The area to volume ratio of this effective portion L e  falls within certain limits which promote more efficient separation, particularly of smaller droplet sizes of the dispersed phase.

FIELD OF THE INVENTION

The present invention relates generally to the separation ofliquid/liquid mixtures, and more particularly, but not by way oflimitation to the separation of oil from oily water or water from oilwhere these liquids are being processed such as an oil well productionsite or any processing plant where oil and water are handled as amixture.

DESCRIPTION OF THE PRIOR ART

In a typical oil well production operation the amount of produced waterincreases as a field matures. In some, if not most operations, the bulkof the volume of produced fluids may be water. Consequently, there is insuch operations, a large amount of water to be disposed of or otherwisehandled in some manner. This typically is an expensive process, i.e.dealing with disposal of produced water. Although there may be no directeconomic incentive, recent tightening of government regulations invarious parts of the world regarding the amount of oil in dischargedwaters has increased interest in improving and optimizing oily waterseparators. This problem is particularly acute on offshore productionplatforms. Size and weight limitations on separation equipment limit theavailable options. Furthermore, on floating offshore platforms, themovement of the platform may effect the performance of some traditionaltypes of separating equipment. Not the least important is the problem ofseparating oil from water to the extent that the water can be disposedof in the sea or body of water within environmental standards.

In order to solve some of the problems associated with the circumstancesdescribed above, hydrocyclone separators have been employed to solvethese oil water separation problems. Traditionally, however, the use ofhydrocyclones has been for the separation of mixtures such asliquids/solids and gas/solids and therefore the problems associated withliquid/liquid separation were not addressed. In particular, theseparation of oil and water further complicates the issues in that theseparation associates with high shear fields which provide an additionalproblematic aspect to liquid/liquid separation. The nature of anoil/water mixture is that of a liquid dispersion which is a complexdispersion, and the application of a hydrocyclone to the separation oftwo immiscible liquids was at one time thought to be impossible. Aliquid dispersion within a continuous liquid phase poses separationproblems to a person skilled in the art, such as the low differentialdensity between the liquid phases and the sensitivity of the liquiddispersion to sheer forces. Generally, the differential density(specific gravity) is of the order of less than 0.2 and presents aproblem when separating the dispersion from a continuous phase. Mosthydrocyclones in present day use are designed for removing a more densedispersion from the continuous phase and do this by creating a vortexwithin the hydrocyclone body which causes the more dense dispersion tomigrate radially outwards towards the walls leaving a dispersiondepleted continuous phase near the axis of the hydrocyclone. To thecontrary, nearly all oils are less dense than water and therefore whenoil contaminated water is passed through a hydrocyclone the radialacceleration of the vortex causes the oil droplets (disperse phase) tomigrate towards the hydrocyclone axis at the center of the vortex,leaving oil free water (continuous phase) near the walls of thehydrocyclone. This, therefore, puts different constraints upon thedesign of the hydrocyclone. Whereas, with a more dense dispersion themajority of the continuous phase is removed through the vortex finder inthe upstream end wall of the hydrocyclone, as the overflow. Theseparated dispersion leaves the cyclone, with a small part of thecontinuous phase from the wall boundary layer, in the underflow. Whenthe dispersion is the less dense phase, the underflow becomes thegreater proportion of the total throughput (90 to 95%) while theoverflow (removing the dispersion from the hydrocyclone axis), is muchreduced. Also, the more dense dispersion upon reaching the hydrocyclonewall is held there in the relatively stable wall boundary layer but theless dense dispersion that forms a core along the hydrocyclone axis hasno such constraint and relies entirely upon the favorable internal flowstructure for its stability and removal from the hydrocyclone withoutfurther disruption. Since the density difference is relatively smallwith an oil dispersion in water, and the hydrocyclone design mustproduce regions of very fast spin to promote separation; but at the sametime, also avoid breakup of the oil drops in regions of high shear. Withthese constraints in mind, the design of an efficient hydrocyclone foroily water separation although perhaps superficially similar to the caseof a hydrocyclone for the more dense dispersion, is essentiallydifferent in its requirement, leading to a rather different geometry.

Important early work in the development of the liquid/liquidhydrocyclone occurred in the late 70's and early 80's by Martin Thew andDerek Coleman at Southampton University and at this stage test workindicated that the minimum nominal size of a hydrocyclone would be about28 mm. It was thought that any further reductions of the nominal size ofa liquid/liquid hydrocyclone would generate high internal sheer stresseswhich would further break up the sensitive oil in water dispersions andtherefore, would work contrary to the desired separation of the phases.This nominal size of the hydrocyclone is referred to as the referencediameter of the cyclone. i.e., the point substantially at which thetangential component of velocity is maximum. Thus, in view of this earlywork and problems in the petroleum industry needing a solution,liquid/liquid hydrocyclones were used commercially for the purpose ofseparating two immiscible liquids, with the majority of theseapplications being for removing relatively small percentages of oil fromwater within the oil industry. In view of this early work by Thew andColeman, the nominal sizes of liquid/liquid hydrocyclones dealing withoil/water dispersions, that have been marketed up to the present, havebeen 35 mm, 38 mm, 58 mm, and 60 mm.

The reference to the nominal size gives an indication of the capacity ofthe unit for a given driving pressure. Generally, from a manufacturingpoint of view the larger the nominal diameter of the cyclone the lessseparation units or tubes that will be required for a given systemcapacity. The above sizes were selected by various designers based onthe aforementioned research, with a view to obtaining optimal separationefficiency and capacity throughput. Generally, as mentioned above, theminimal diameter was thought to be 28 mm and no liquid/liquidhydrocyclone with a nominal diameter less than 30 mm were marketed forthis reason. Some of the practical problems associated with operatingthe liquid/liquid hydrocyclones were that the separation efficiencyachieved with the nominal diameter cyclones ranging from 35 mm to 60 mmwould not be sufficient to meet certain process specifications. Someprocesses have relatively small volumes of liquid that require a liquiddispersion to be separated and may be typically less than for example,100 barrels a day in an oil field operation. The minimum volumetric flowrate, under which a single 35 mm hydrocyclone can perform adequateseparation, is typically greater than 400 barrels per day. The 60 mmnominal size cyclone requires at least a thousand barrels a day.

Another problem relates to the minimum driving pressure. For ahydrocyclone having a nominal diameter of 35 mm, the minimum drivingpressure is about 60 psi, and for a 60 mm hydrocyclone, it is about 100psi. There are other factors that may effect the minimum drivingpressure, such as the viscosity of the continuous phase.

To further digress into the background of this invention, prior to thework set about by Thew and Coleman in the late 1970's, the standarddesign of a hydrocyclone was for solid/liquid or solid/gas applicationsand included a short cylindrical inlet section followed by a relativelysteep tapered cone. The overall length/diameter ratio of the separationchamber, typically, was about ten. Work was done in the mid-1960's byboth Regehr and Bohnet on liquid/solid technology who, eventually alongwith Thew, Coleman, and Listewnik began to work with liquid/liquidproblems. Because of shear sensitivity in oil/water separation problems,it was initially concluded that it was necessary to impart centrifugalforce under much gentler conditions and therefore, the only way togenerate proper separation was to increase the length/diameter ratio.These early researchers took as their starting point a cylindricalcyclone having a length/diameter ratio between 10 and 25. The result wasa simple cylindrical chamber with Regehr adding a small cone to theoutlet side. Their early work found that going beyond a 25length/diameter ratio was anti-productive in that the frictional loss inlonger chambers would be significant enough that the tangentialcomponent of motion would be severely reduced and therefore, there wouldbe a loss in flow to the extent that the flow through the hydrocyclonewould go to a laminar flow regime. Therefore, it developed that thelength/diameter ratio of 10 to 25 was the optimum. Then, Thew, in thelate 70's, started working with a design which had multiple, decreasingdiameter steps in it. The concept was that (1) it was desirable toincrease the length of the separation unit in order to increase theresidence time and also to gradually increase or at least maintain thevelocities throughout the hydrocyclone period. This was done bydecreasing diameters. The original work was simply a design whichmaintained angular momentum over the frictional loss as in the cyclone.These designs eventually evolved to a hydrocyclone having alength/diameter ratio of 10 to 25 and a nominal diameter d₂ which was0.5 d₁, with d₁ being the diameter of the inlet portion. There was thenprovided a taper from D₁, of approximately 10° half-angle, up to theportion d₂, which was considered the nominal diameter of thehydrocyclone, at which point the tangential velocity was near a maximum.Eventually, the overall length/diameter ratio approached 25, in thatThew's work showed that better separation was attained by maintainingthe tangential velocity component over a greater period of time. Workdone by Thew and Coleman in 1981 demonstrated that the efficiency ofseparation would increase with the use of smaller diameterhydrocyclones, however, because of problems associated with highshearing in oil/water dispersions, their work noted as previouslymentioned that a minimum nominal diameter limitation of 28 mm prevailed,that smaller diameters would increase oil drop breakup, and thathydrocyclones below this 28 mm size were too small for good efficiency.

Hydrocyclones began to find commercial acceptance in the oil/watermarket because of the increased attention to environmental pollution. Inorder to return produced water to the ocean in offshore operations, itbecame mandatory to reduce the residual oil in water to typically lessthan 40 ppm. Many separation situations have been able to utilize theheretofore described hydrocyclones to accomplish this task. Suchhydrocyclones are described in U.S. Pat. Nos. 4,237,006 and 4,257,368 toColeman and Thew. It is interesting to note that these and other patentssuggest that the nominal diameter of a hydrocyclone can range from 5 to100 mm, although later research data by these same inventors indicatedthat hydrocyclones below 28 mm would be inefficient. It appears thatthese earlier suggestions of nominal diameters ranging up from 5 mm weremerely speculative and had no real basis other than an attempt tobroaden the disclosure to cover all the possibilities, in that no knownattempts were made to build and test, at least on a commercial basis,such smaller models. In any event the work by Thew and Colemanindicating that diameters less the 28 mm would be inefficient wassufficient to discourage any further reduction of sizes below about 30mm. Lengths were disclosed as being under 25 times the nominal diameter.The same three portion geometry disclosed in these patents and otherequivalent configurations including curved housings, have dominated thecommercial oil/water separation market for separators in the petroleumindustry. U.S. Pat. Nos. 4,544,486; 4,464,264; 4,719,014 and subsequentpatents to Noel Carroll further described refinements to the basic orstandard "deoiling" hydrocyclones. Later patents such as U.S. Pat. No.4,721,565 to Carroll and U.S. Pat. No. 4,749,490 to Smyth and Thewapplied variations on those geometries to apply this technology to theseparation of oil and water, where oil represents a greater percentageand may be even the major portion of a mixture. These are sometimestermed "dewatering" hydrocyclones. In some of these mixtures, water maybe the disperse phase in an oil continuous phase.

As industry acceptance of this product has grown, so has the need formaking the device more efficient in separation of the phases. Indeoiling operations, the need is being seen to reduce the percentage ofoil in oily water being separated for discharge back into the ocean.Also, separation applications which will not respond to these abovementioned systems now bear looking at if more effective and efficienthydrocyclone separators are produced.

Therefore, in order to meet the ever increasing more stringentrequirements of the environmental regulations, and separation problemsunsolvable by present commercial products, it is becoming increasinglyimportant to develop separation techniques of the type which willprovide more efficient separation in order to meet the criteria which,for example, in the petroleum industry is now tending to fall below theheretofore typical standard of 40 ppm of oil in water which is to bereturned to the ocean in an offshore drilling operation. As a result,testing has recently been established to determine whether theheretofore size barriers which were considered as limiting, wouldprovide a solution to the overall problem of increasing the efficiencyof hydrocyclones in oil/water separation. This recent work has led tonew developments in hydrocyclone design which appear to have broken somerestrictive performance barriers in prior commercial designs. One suchdevelopment is a hydrocyclone utilizing a very rapid acceleration designtogether with a minimal length inlet chamber and progressively reducingin size through a rapid transition down to the nominal diameter size ofthe hydrocyclone. This concept is based on the theory that you stabilizean inlet flow in a very small chamber by maintaining an optimalcondition where the inlet velocity at the inlet diameter d_(i) does notexceed the sheer stresses of the droplets so that you get a goodtransition acceleration of the flow entering the hydrocyclone withoutshearing it at this point. Once you reach this optimal stabilizedcondition, you can then very rapidly decrease the diameter of theseparation chamber which in turn increases the acceleration field. Thiswork tends to show that the sooner you narrow to a nominal diameter d₂,the higher the efficiency of the unit. In following this theory youtransition from diameter d.sub. i to diameter d₂ rapidly and then use avery gradual taper from d₂ to the outlet diameter. On the other hand,with such a rapid transition you create significantly higher pressurelosses in the cyclone and the increase in pressure loss because of thehigher acceleration field gives a lower flow capacity through thecyclone which provides a concern from the capacity standpoint. In orderto meet flow requirements it is then necessary to increase the number ofcyclones in a system.

In the design of liquid/liquid hydrocyclones it has been consideredimportant to provide very smooth transitional flows along the length ofa hydrocyclone in an attempt to maintain a given centrifugal force for agiven length of the cyclone with a minimum of frictional losses and flowdisruptions. In an attempt to improve separation efficiency, it wasdecided to experiment with a simple extension to the separation chamberlength to allow additional residence times so that smaller dropletswhich are less buoyant can effectively have more time to reach thecentral core to be removed. Prior attempts have typically been around amethod to have a smooth transition from the inlet flow to a maximumacceleration, maintaining that acceleration for a certain length, andthen adding residence time as needed. The trade-offs here are that theadditional residence time creates considerable additional pressure dropwhich in the past was felt would be unwarranted due to marginalincreases in efficiency that would be gained. Another aspect to havingrapid transitions between accelerations in the cyclone was that byincreasing the fluid velocity or angular momentum rapidly there would besheer stresses created in the fluid flow and therefore the droplets ofthe dispersed phase would be harder to remove. It is noted that in theinlet design you typically attempt to increase the velocity entering thecyclone only to a point where shear stresses becomes excessive anddroplet breakup becomes significant. Significant breakup means thatdroplets are sheared to a point that they can not be effectively removedin the cyclone. Therefore, in order to solve the problems at hand thereare basically two issues. One concerning the effect of droplet shearingdue to acceleration which is a major concern and a second issue havingto do with the effect of increasing the pressure loss in thehydrocyclone. Pressure loss is detrimental with respect to availablepressures to operate the cyclone, which may have to be increased andtherefore the energy input would have to be increased.

Another problem which was faced by designers of water/oil hydrocycloneswas that of "reintrainment". When oil migrates to the core it can becomereintrained into the water in a boundary layer that exists near thecore. This is thought to be especially true if the velocity of swirl islow. However, by increasing the velocity you also incur the risk ofincreasing shear. Thus, while smaller diameters and steeper tapers mightincrease the velocity of swirl, they would also increase the prospect ofshear of larger droplets.

Yet another phenomenon which occurs with steep taper is that ofrecirculation. Eddy currents in the inlet section tend to build up whenyou reduce a large volume to a small volume in a short distance. Theconstriction provides a flow vector toward the inlet which promotes anaxially oriented swirl. This phenomenon was seen by Thew and others andsteered design to gentler tapers. Recirculation in the involute gives abuildup of solids in the inlet end section near the outer wall where itmeets the inlet end which in turn causes erosion in the chamber.

Thus, notwithstanding, the apparent adverse phenomena attributed todecreasing the diameter of a hydrocyclone as set forth above, applicantsnevertheless made an attempt to overcome the obvious problems in orderto develop a higher efficiency hydrocyclone. This decision was partlymotivated by the realization that past work had been done with largedroplet sizes and now the applicants were again looking at droplet sizedistribution and more carefully noting that distributions nearly alwayscontained a segment of smaller droplet sizes which would ultimately haveto be dealt with. While it was understood that a smaller hydrocyclonewould develop high acceleration fields and higher velocity gradientsthus tending to increase the turbulence level as related to particleintegrity, a more careful look at the dynamics of a hydrocycloneseparation chamber show that the smaller droplets, for example, in the 5to 10 micron range tend to be more robust and less effected by shear.Again, in solid particle hydrocyclones you can go to a smaller diameterseparation chamber without detriment due to shear considerations in thatwith particles you want a higher acceleration field, and shear can aidin separation by knocking out particles. It occurred to the presentinventors that a window would occur for separating out small droplets inthe liquid/liquid field if, in fact, you manage the small droplets insuch a way as to not encourage the further shearing of the droplets, atleast beyond a level that can be dealt with in the hydrocyclone. It wasdecided to work in this direction with the ultimate hope that ahydrocyclone of a smaller diameter could be developed which wouldincrease the efficiency of liquid/liquid separators and in particular anoil/water separator involving dispersions having small droplet sizes inthe disperse phase.

It is therefore an object of the present invention to provide a new andimproved hydrocyclone for separating liquid constituents of a mixtureand in particular oil and water phases of a liquid mixture wherein thenominal diameter of the hydrocyclone, or throat diameter as it isreferred to herein, is in the range of 8 to 28 mm. The improvedhydrocyclone would be capable of operating at low driving pressures andstill maintain satisfactory separation efficiency, notwithstanding thesmall diameter, where the minimum volumetric capacity can be as low asabout 50 barrels a day for a single cyclone unit. Such an improvedseparator will be effective to process small droplet sizes and will havegreater efficiencies than that of larger nominal diameter cyclones atsimilar process conditions. The area to volume ratios utilized in theelongated portion of the separation chamber in such a hydrocyclone willbe in a range of values which promotes an unusually high efficiency.

Summary of the Invention

With these and other objects in view the present invention contemplatesa hydrocyclone for separating liquid phases of a mixture wherein one ofthe constituents is a disperse phase of liquid droplets within a liquidcontinuous phase such as occurs in an oil in water or water in oildispersion. The hydrocyclone comprises a separation chamber having oneor more inlet openings for inletting the fluid mixture into an inletportion at one end of the separation chamber. An overflow outlet, whichmay be axially disposed in the end wall of the inlet portion, providesan outlet for a less dense phase of the mixture. An underflow outlet foroutletting a more dense phase is disposed at the other end of theseparation chamber opposite the inlet end. The separation chamber isfurther characterized by a first relatively steep tapered or curvedportion for rapidly accelerating the fluid within the accelerationchamber, yet without further shearing the disperse phase droplets beyondthat level than can be handled by the hydrocyclone. The relatively steeptapered first portion is followed by a less steeply tapered or curvedsecond portion, which represents an elongated portion of the separationchamber. The transition between the first and second tapered portions isrepresented by a throat portion having a diameter D_(T) <28 mm which isalso referred to as the nominal diameter of the hydrocyclone. Theseparation chamber can alternatively even be cylindrical or partiallycylindrical if it meets the functional needs of the invention. Theelongated portion of the separation chamber extending past D_(T)includes any tapered, curved, cylindrical or substantially cylindricalportion which makes up the total length L_(e) of the elongated chamberup to the underflow outlet. This elongated portion L_(e) of theseparation chamber is shown to operate most efficiently for suchliquid/liquid mixtures when arranged to have an area to volume ratio(A/V)_(L).sbsb.e falling within certain parameters. This efficiency andthe overall performance of the hydrocyclone is further enhanced bylocating the D_(T) within a certain limited distance from the inlet. Themean droplet size that is removed by such a hydrocyclone is reduced inproportion to increases in efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (a and b) is a schematic view of two alternative geometricalshapes for separation chambers in a hydrocyclone;

FIG. 2 is a graphical representation of flow rates versus delta pressurebetween inlet and outlet of various nominal sizes of hydrocyclones;

FIG. 3 is a graphical representation of inlet pressure versus efficiencyfor a standard and small nominal diameter hydrocyclone;

FIG. 4 is a graphical representation of inlet/outlet pressuredifferential versus increase in efficiency for various nominal sizes ofhydrocyclones;

FIG. 5 is a graphical representation of nominal diameter of hydrocycloneversus area to volume ratio versus improvement in efficiency; and

FIG. 6 is a graph showing a normalized curve representing drop sizedistribution of a dispersed phase of an oil-water mixture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1 (a and b) of the drawings, a hydrocyclone 12is shown in two slightly different configurations, first having anelongated separation chamber beginning at a larger end 14 and continuingin a housing formed by a volume of revolution to an opposite smaller endforming an underflow outlet 16. An overflow outlet 18 is shownpositioned in the wall of the larger end 14. An inlet 20 at or near thelarger end 14 provides an inlet means for fluid mixtures into theseparation chamber. An inlet diameter D_(i) represents the averagediameter at which flow enters the cyclone through the one or more inletsand is thus twice the minimum distance of the tangential component ofthe inlet centerline from the cyclone axis. The inlet plane is definedas the plane perpendicular to the axis of the cyclone at the mean axialposition of the inlets such that the injection of angular momentum offluids into the hydrocyclone is equally distributed about it.

It is to be noted that the profile of the volume of revolution formingthe separation chamber may take an infinite variety of forms with onlytwo such configurations being set out in FIG. 1 (a and b). The FIG. 1ais comprised of discrete portions including an inlet portion 22represented by a cylindrical chamber followed from left to right by afirst steeply tapered conical portion 24 which is then followed by asecond less steeply tapered conical portion 26. The transition betweenthe first and second tapered portions is represented by a throat portion28 which is also styled as D_(T) for reference purposes. This D throatis also referred to as the theoretical nominal diameter of thehydrocyclone and represents an important reference for teaching theconcepts embodied herein which provide high efficiency characteristics.It is to be remembered that this D_(T) is merely a theoretical referenceand while it is purported to be the place where approximate maximumtangential velocity or swirl occurs, it may be in reality that such apoint would be difficult to pinpoint precisely. Therefore, it is thepurpose herein to use the concept to transmit the teaching embodiedherein and to provide sufficient certainty to claim the invention;however, it should not be taken as an absolute. The final portion of theseparation chamber in FIG. 1a is shown as a cylindrical or substantiallycylindrical parallel section 30.

In FIG. 1b, the separation chamber shown does not display discreteportions but nevertheless such portions are embodied at leastfunctionally in FIG. 1B. For example, where the inlet 20 enters theseparation chamber, there is functionally an Inlet portion correspondingto the inlet portion 22 in FIG. 1d, etc. with the other correspondingportions being represented throughout the length of the entireseparation chamber as it progresses in a continuous curve to theunderflow outlet 16. The throat diameter D_(T) which is also referred toas the nominal diameter is defined in several different ways as will behereinafter described, but functionally, the throat is that approximateplace where the fluids flowing through the separation chamber havesubstantially reached their maximum acceleration and tangentialvelocity. As previously stated this is not an absolute. In the standardgeometry that has developed in the liquid/liquid hydrocyclone art, d₂ isusually the designated reference for this nominal diameter, hereindesignated D_(T). d₂ is sometimes defined as occurring where thediameter of the body is, 0.5 d. In the case of a single involute inletD_(i) is generally greater than D₁ but in the case of a straight turntangential inlet D_(i) can be less than D₁. In this application D_(i)represents a functional place where the mixture enters the hydrocycloneand is therefore descriptive of what is happening dynamically.Therefore, in this application D_(T) is described with reference toD_(i), usually as occurring at >0.5 D_(i) or equal to 0.6 D_(i).

As was discussed in the Background of The Invention, it is a change inconceptual thinking that now leads to the implementation of a smallerthroat diameter which is an important aspect of the present invention. Agreat deal of experimental work has recently been done to identify thoseparameters including optimal nominal diameters which provide an increasein separation efficiency. It has been found that efficiency generallyincreases as the diameter decreases. As diameter decreases, there isalso an accompanying increase in pressure drop across the unit assumingconstant throughput, or a decrease in throughput assuming a constantinlet pressure. FIG. 2 shows graphically what occurs to flowrate versuspressure when using three different sized hydrocyclones built with ageometry configured similar to FIG. 1a. By picking any point of constantpressure differential between inlet and outlet pressure, say 50 psi forexample, it is seen that for a nominal diameter 19 mm hydrocyclone theflowrate is approximately 10 gallons per minute while at the same deltaP, the flowrate is approximately 29 gpm for a 35 mm hydrocyclone. This,of course, illustrates the disadvantage of small flowrates with asmaller diameter unit, if large flowrates need to be dealt with, whichwould necessitate a plurification of individual separation chambers. Onthe other hand, it is seen in FIG. 3 that for a given inlet pressure a12 mm hydrocyclone demonstrates a substantial improvement in efficiencyover a 35 mm unit. The tests which were used to gather this data hadparallel flow lines arranged so that produced fluid from a well wasdivided through two flow lines simultaneously to be separated in the twodifferent separation chambers, i.e., 35 mm and 12 mm sizes. Thisprovided identical conditions for all the liquid properties includingdroplet size distribution, density, temperature, etc.

While it is obvious that utilizing a smaller diameter will increasepressure at a given flowrate within a hydrocyclone, it is also expectedthat the increase in pressure will increase the likelihood of shearstresses being imposed upon the droplets making up the disperse phase.In considering design parameters of hydrocyclone, we have in the pastfocused on Stokes law which in part says that separation is a functionof droplet size and that small droplets are significantly harder toseparate than large droplets. Thus, hydrocyclones will not efficientlyremove smaller droplets within a certain range. It was also thought thatsmaller diameter cyclones and steeper taper angles in the separationchamber were likely to shear droplets within the cyclone, thusgenerating smaller droplets which would be difficult to remove.Therefore, designers failed to take advantage of the fact that smallerdiameter cyclones effectively remove smaller droplet sizes. This isbecause the effect of shearing in the cyclone is less detrimental whenyou are dealing with smaller droplets which are already sheared, so tospeak. Therefore, shearing in the cyclone caused by design parameterswill tend to mostly affect the larger droplets, i.e., it is the largerdroplets that will be sheared in the cyclone. However, as long as wedon't shear below say 6 microns, which the smaller diameter units willaccommodate, then we need not be concerned with the fact that bothsmaller throat diameters and more rapid transition to smaller D throatwill likely cause shearing of larger drops, since these sheared dropswill now be more effectively removed. This also mitigates against theteachings of shearing problems with respect to reintrainment andrecirculation. In view of the above, it is now recognized that thehydrocyclone may be configured to quickly accelerate the mixture to takeout the larger droplets and again quickly accelerate the remainingmixture without further shearing what is left, or not shearing whatremains below say 6 to 8 microns. With the smaller diameter, thehydrocyclone has a smaller volume and the remaining droplets will have abetter chance to contact one another, thereby coalescing, and thus bemore likely to separate due to the centrifugal separation process of thehydrocyclone. By having a smaller diameter, you also physically reducethe distance that a droplet must migrate to reach the central core andthereby also reduce the time that it takes to reach the core. Thesecharacteristics lead to an increase in the range of droplets that areremoved from the mixture and thus increase the efficiency of separation.If we then add a longer residence time for maintaining this dynamic, thechance for coalescence and thus separation is further enhanced.

While it is recognized that the smaller diameter and/or more steeplytapered hydrocyclone will most likely generate additional shear of thelarger droplets, this disadvantage appears to be outweighed by the factthat the smaller diameter hydrocyclone also will separate out a smallerdroplet size and thus the mean droplet size removed decreases with thenominal size of the hydrocyclone. Therefore, the volume of dropletsremoved is increased in smaller nominal sizes and efficiency is therebyincreased. Data suggests that the hydrocyclones having a nominaldiameter of 8 to 28 will effectively remove droplets in the 4 to 10micron range. Thus, while shearing of droplets may take place as aresult of the smaller size and steeper tapers, this effect is offset bythe increased portion of the mixture that is removed. For a given set ofconditions, the underflow outlet will have disperse phase componentspresent in the form of droplets which have not separated out in theseparation chamber. With the smaller nominal size diameter body thedroplet size distribution of these remaining droplets at the underflowoutlet will have a mean size which is less than the range of 4-8 micronwhereas a larger than 28 mm hydrocyclone under the same conditions willhave an outlet distribution that is higher. That portion of the mixturewhich is represented by this differential in mean droplet size will bedeterminative of the improvement in separation efficiency. Thisrelationship is shown graphically in FIG. 6 wherein the normalized inletdroplet distribution of a mixture is represented by the area under thecurve in that figure. For purposes of illustration the mixture has adisperse phase of 1000 ppm of oil in a continuous phase of water, which1000 ppm of the disperse phase is represented by the area under thecurve. The mean droplet size D₅₀ of the disperse phase in thisillustrative example is 20μ. If a 35 mm nominal diameter hydrocycloneremoves these droplets under a given set of conditions (temperature, ph,etc.) above those having a mean size of 8μ and that portion of thedisperse phase above 8μ represents 80% of the distribution. D₈₀. thenthe hydrocyclone efficiency is 80% and the droplet concentration at theunderflow outlet will be 0.2×1000 ppm or 200 ppm. A 20 mm nominaldiameter hydrocyclone might, however, remove droplets down toapproximately 4μ under these same conditions, lets say down to a meansize of 6μ, which represents 90% of the distribution, D₉₀. Therefore,the performance of this latter hydrocyclone will be 90% and 1000 ppm x0.1=100 ppm, thus providing an increase in efficiency of 10%. Early dataindicates that under normal field operating conditions, the illustrativeexample above is representative of the improvement that can be expectedwith the smaller nominal diameter units. This, of course, presupposesthat the mixture contains a substantial portion of smaller dropletswhereby this advantage will operate.

FIG. 4 provides a comparison for various sized separation chamberbodies, for percent increase in efficiency over the 35 mm body atvarying inlet/outlet delta P. This data was taken, as earlier described,where a small diameter unit was run in a parallel flow loop with a 35 mmunit. The mixture contained a mean drop size of the disperse phase in arange of 12-14 micron and the mixture temperature was 112° F. It can bereadily seen from Figure A that a direct correlation exists between bodysize and efficiency. It is also interesting to note in FIG. 4 that oneof the top lines marked 15 mm ASAD has a body similar to the lower linedenoted as simply 15 mm, except that the hydrocyclone body representedby the upper line has an extended parallel section added to the body toafford a greater residence time to the fluids in the separation chamber,and consequently, has a higher efficiency.

In analyzing the vast amount of data taken in the course of these tests,a theme or rational was sought for the various phenomena describedabove. While an absolute explanation is perhaps not possible, a likelyexplanation has been set forth above relative to longer residence time,shorter distances for drops to travel, behavior of small droplets underhigh shear conditions, etc. However, in further analysis it appearedthat a heretofore unknown relationship existed which impacts greatly onthe problem at hand, that being the area to volume ratio of theseparation chamber after it reaches a place in the body shape wheremaximum acceleration occurs. This place we believe occurs approximatelyat that place in the standard type configurations as set forth in FIG. 1where D_(T) occurs, that also being where the slope changes from a steepslope to a finer or more gentle slope whether it be a taper or theequivalent curve. In this respect, the following illustrates how thisA/V ratio can be calculated for a given configuration. ##EQU1##

For components with circular cross sections: ##EQU2## where: X=distancealong centerline of component

r_(c) (x)=radius as a function of axial distance for component c

c=component number 1, 2, 3 . . . , n

i=inlet

o=outlet

The function r_(c) (x) is completely general and takes on forms like thefollowing, but not limited to the following:

(1) For a cylindrical component:

    r.sub.c (x)=k

where k is a constant radius

(2) For a conical section:

    r.sub.c (x)=r.sub.i Δ(x)

    γ(x)=x sin θ

so

    r.sub.c (x)=r.sub.i -x sin θ

where θ is the half-angle of the cone

(3) For a quadratically curved component:

    r.sub.c (x)=r.sub.i +βx+αx.sup.2

where =β and α are constants describing the

curvature of the component

In order to calculate the A/V ratio of the total body configuration inof FIG. 1a you simply integrate the area and volume of each section ofthe body taken separately. For the purposes of this invention, it is thearea to volume ratio of that portion L_(e) of the separation chamberbeginning at L_(T) and extending to the underflow outlet, that is ofinterest. Thus, for the configuration shown in FIG. 1 (a and b) we wouldfirst calculate the area and volume of that portion occurring from D_(T)to the dotted line 34. Then you would calculate the area and volume ofthat portion 30 extending from dotted line 34 to the outlet 16, which inthis case is shown as a cylindrical component. This latter portion, ofcourse, would have a constant radius and would not need be integrated,and the area for example, would simply be calculated, as 2 πrx, with xbeing the axial length of the component portion involved. Volume in thissame cylindrical portion would be calculated as πr² x. This procedure isfollowed then until the area and volume of all portions of interest arecalculated. It should be noted that this portion L_(e) is a functionalportion of the chamber and since there are many configurations ofchambers that can be devised, it is well to understand this teaching inits functional terms. The beginning of the portion L_(e) is where it isapproximated that the maximum velocity and maximum acceleration of thefluid occurs. This is not an absolute place from a practical standpoint,at least as to being able to determine its precise location. If thechamber were a straight taper from beginning to end; i.e., a truncatedcone; having a single angle of taper, this D_(T) would be described asoccurring at the inlet to the chamber and L_(e) would begin at D_(i)near the end wall 14. As to the underflow outlet or end of L_(e), thisoccurs where the separation process ends, at the outlet thereof. If onewere to add a diverging section of some length to the end of theseparation chamber it is likely that radial acceleration or swirl in thechamber would drop off and thus separation due to centrifugal forceswould end. This would functionally be the end of the separation chamberand the end point of distance L_(e). Where D_(T) has been defined tooccur at some certain point, it is for purposes of being definite, asfor example, in claiming the invention. A more functional way to fixD_(T) is within some range in which it will fall. Another way todescribe D_(T) is at a place where the slope of the chamber wall reachesa certain value.

Reference is now made to FIG. 5 of the drawings which shows a plot ofArea to Volume ratios versus throat diameters for various configurationsof hydrocyclones. The percent of approximate improvement over a 35 mmhydrocyclone is shown on the right hand scale. The percent improvementwill also be a function of droplet distribution, temperature, density,ph, interfacial surface tension, etc. The lower curve is for a body suchas shown in FIG. 1a with L_(e) beginning at D_(T) and with no parallelextension. The parallel extension is that portion of the separationchamber that is cylindrical or substantially cylindrical and extendsfrom the dotted line 34 to the underflow outlet 16. Thus, the bottomcurve of FIG. 5 is represented by a body having an L_(e) that ends at 34as shown in FIG. 1 (a and b). The middle curve of FIG. 5 (a solid line)represents a hydrocyclone similar to the standard geometry of FIG. 1ahaving a 0.75 degree taper angle α for the tapered portion 26 as shownat 36. This unit also has a parallel extension portion 30 of 20 _(DT).The upper curve of FIG. 5 represents a hydrocyclone body having a taperangle of 3° and a parallel extension of 100 D_(T). It is seen from thedata presented in FIG. 5 that the area to volume ratio for hydrocycloneshaving a throat diameter of 8 to 28 mm generally falls within a range of0.19 to 0.9. However, there may be petroleum applications not presentlycontemplated or uses in other industries where smaller units would bepractical and therefore the lower limits of data herein stated do notlimit the concepts involved in this teaching to any particular minimumsize.

Therefore, while particular embodiments of the present invention havebeen shown and described, it is apparent that changes and modificationsmay be made without departing from this invention in its broaderaspects, and therefore, the aim in the appended claims is to cover allsuch changes and modifications as fall within the true spirit and scopeof the invention.

What is claimed is:
 1. A high efficiency hydrocyclone apparatus forseparating liquid constituents of differing densities from a fluidmixture wherein one of the constituents is a dispersed phase of liquiddroplets within a liquid continuous phase, wherein the high efficiencyhydrocyclone comprises:a separation chamber having an inlet portion atone end of the separation chamber; inlet means at said one end thereoffor inletting the fluid mixture into the inlet portion of saidseparation chamber to generate a swirling motion of the fluid mixture,said inlet portion having an inlet diameter D_(i) in the plane of theinlet means opening into said inlet portion; an overflow outlet on theseparation chamber for outputting a less dense constituent of the fluidmixture; an underflow outlet at the opposite end of the separationchamber from said inlet means for outletting a more dense constituent ofthe fluid mixture; said separation chamber for further comprising apartially tapered, curved, or generally cylindrical outer wall portionwhich begins at or reduces in diameter to a nominal diameter D_(T) ;said separation chamber further having an elongated chamber portionbeginning at D_(T) and extending therefrom which includes any tapered,curved or substantially cylindrical portion making up the total of theelongated chamber portion of said separation chamber from D_(T) up tothe underflow outlet therefrom, such elongated chamber portion beingdescribed herein as having an effective length L_(e) and providing aconfined residence portion wherein throat diameter D_(T) is sufficientlysmall so as to confine the swirling fluid mixture such that the smalldroplets of the disperse phase are more effectively removed through theoverflow outlet; wherein the area to volume ratio of the elongatedchamber portion (A/V)_(Le) beginning at D_(T) and extending the distanceL_(e) is defined as the sum of internal surface areas A of all theportions making up L_(e) of said separation chamber divided by the sumof the internal volumes V of all the portions making up L_(e) of saidseparation chamber, wherein (A/V)_(Le) is greater than 0.21 mm⁻¹ so thatresidence time of swirling liquids in the residence portion is prolongedin a sufficiently confined cross-sectional area and within as small avolume as practical to thereby enhance the efficiency of separation ofthe small disperse phase droplets from the liquid continuous phase; andwherein D_(T) occurs at that place in the separation chamber where theangle of taper α, an angle between a longitudinal axis of the separationchamber and the outer wall of the separation chamber, is less than orequal to 2°, where ##EQU3## and where Δr is the difference in radius ofthe separation chamber over an axial length L of the separation chamber.2. The hydrocyclone apparatus of claim 1, wherein D_(T) is from 8 to 28mm.
 3. The hydrocyclone apparatus of claim 2, wherein L_(e) is greaterthan 7.16 D_(i).
 4. The hydrocyclone apparatus of claim 1, wherein D_(T)is equal to or less than 25 mm.
 5. The hydrocyclone apparatus of claim 1wherein (A/V)_(Le) is from 0.21 to 0.9 mm⁻¹.
 6. The hydrocycloneapparatus of claim 1 wherein D_(i) is less than 70 mm.
 7. Thehydrocyclone apparatus of claim 1 wherein L_(e) is greater than 14.32D_(i).
 8. The hydrocyclone apparatus of claim 1 wherein L_(e) is greaterthan 25 D_(i).
 9. The hydrocyclone apparatus of claim 1 wherein asubstantial portion of the dispersed phase of the liquid mixture to beseparated by the hydrocyclone apparatus is in the form of droplets lessthan 20 micron in size and the apparatus herein is arranged to separatefrom the mixture substantially all those droplets having a mean sizegreater than 4 micron.
 10. The hydrocyclone apparatus of claim 1 whereinthe hydrocyclone apparatus herein is arranged to remove a significantlygreater portion of dispersed phase droplets from the more denseconstituent than would be removed from a mixture under the sameconditions with a hydrocyclone having a nominal diameter equal to orgreater than 30 mm.
 11. The hydrocyclone apparatus of claim 1 whereinthe apparatus herein will separate substantially all dispersed phasedroplets having a mean size greater than 6 micron from the more denseconstituent of the mixture.
 12. The hydrocyclone apparatus of claim 1wherein the apparatus herein will separate substantially all dispersedphase droplets having a mean size in the range of 2 to 8 micron from themore dense constituent of the mixture.
 13. A high efficiencyhydrocyclone apparatus for separating liquid constituents of differentdensities from a fluid mixture wherein one of the constituents is adispersed phase of liquid droplets within a liquid continuous phase,wherein the high efficiency hydrocyclone comprises;a separation chamberhaving an inlet portion at one end of the separation chamber and anouter wall portion throughout the length of the separation chamber;inlet means in the inlet portion for inletting the fluid mixture intothe inlet portion of said separation chamber to generate a swirlingmotion of the fluid mixture; an overflow outlet axially positioned atsaid one end of the separation chamber for outletting a less denseconstituent of the fluid mixture; an underflow outlet at the oppositeend of the separation chamber from said inlet means for outletting amore dense constituent of the fluid mixture; said separation chamberfurther comprising a first relatively steeply tapered or curved portionand a second elongated less steeply tapered, curved or substantiallycylindrical portion, said first portion connecting the inlet portionwith the second tapered, curved or substantially cylindrical portion toprovide a section for accelerating the swirling motion of the fluidmixture, with the transition between said first and second portionsbeing represented by a throat portion having a throat diameter D_(T)which is located at a point in the separation chamber at or near saidtransition between said first portion and said second portion where anangle of taper α between a longitudinal axis of the separating chamberand the outer wall portion is equal to 2°, where ##EQU4## and where Δris the difference in radius of the separation chamber over an axiallength L of the separation chamber; where D_(T) is less than or equal to28 mm to thereby be sufficiently small to bring the accelerated swirlingmotion of the fluid mixture into a sufficiently confined residenceportion of the separation chamber beginning at D_(T) to enhance theseparation of small droplets of the disperse phase from the mixture,with D_(T) being located at a distance L_(T) from said one end which isthe distance from D_(i) to D_(t) ; the second portion beginning at D_(T)and extending therefrom including any additional tapered, curved orsubstantially cylindrical portion which makes up the elongated portionof said separation chamber extending up to the underflow outlettherefrom located at the opposite end of said separation chamber; saidelongated portion beginning at throat D_(T) being described herein ashaving an effective length L_(e) which is greater than 37.5 D_(T) toensure that the small droplets of the dispersed phase are givensufficient time in the confined residence portion to provide forenhanced coalescence of the small droplets in the fluid mixture; whereinthe area/volume ratio (A/V) of the elongated portion beginning at D_(T)and extending the distance L_(e) is defined as the sum of internalsurface areas A of all the portions making up L_(e) of said separationchamber divided by the sum of the internal volumes V of all the portionsmaking up L_(e) of said separation chamber, and wherein (A/V)_(Le) isgreater than 0.21 mm⁻¹ so that the residence time of the acceleratedfluid is prolonged in a sufficiently constricted cross-sectional areaand within as small as a volume as practical in the residence portion tothereby further enhance the efficiency of separating the small dropletsof the dispersed phase from the continuous phase; and wherein asubstantial portion of the disperse phase of the liquid mixture to beseparated by the hydrocyclone apparatus is in the form of droplets lessthan 20 micron in size and the apparatus herein is arranged to separatefrom the mixture substantially all those droplets having a mean sizegreater than 6 micron.
 14. The hydrocyclone apparatus of claim 13wherein L_(T) is equal to or less than 2.41 D_(i) for more rapidlyaccelerating the swirling motion of the fluid mixture in a relativelyshort first portion.
 15. The hydrocyclone apparatus of claim 13 whereinL_(e) is equal to or greater than 49 D_(T).